Hydraulic elevator control system



March 17, 1964 D. J. ARBoGAsT ETAL 3,125,319

HYDRAULIC ELEVATOR CONTROL SYSTEM 2N VEN TORS.'

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' HYDRAULIC ELEVATOR CONTROL SYSTEM INVENToRs.-

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l l mi @fw/1% United States Patent HYDRAULIC ELEVATOR CONTROL SYSTEM Duane l. Arbogast, Olive Branch, Mms., and Lawrence F.

Jaseph, Memphis, Tenn., assigner-s to Dover Corporation, Memphis, Tenn., a corporation of Delaware Original application Aug. 31, 1956, Ser. No. 607,496, now

Patent No. 2,953,902, dated Sept. 27, 1960. Divided and this application Aug. 11, 1960, Ser. No. 48,988

4 Claims. (Cl. 251-29) The present invention relates to improvements in hydraulic elevator control systems, and more particularly to a system whereby the speed of the elevator may be more nicely controlled and the elevator may be brought to a stop at the landing with greater accuracy than heretofore possible. The present invention relates to mprovements over the inventions disclosed in the prior issued patents to Lawrence F. Iaseph No. 2,355,164, dated August 8, 1944, and No. 2,553,045, dated May 15, 1951.

This application is a division of copending application Serial No. 607,496, filed August 31, 1956, entitled Hydraulic Elevator Control System, and issued into Patent No. 2,953,902, dated September 27, 1960.

In the hydraulic control systems of the prior patents, and particularly the later issued patent, it was disclosed that the coasting distance to a full stop could be reduced by causing a bypass valve connected to the pump discharge to open at the same time the pump motor was deenergized, thus reducing the variation in the coasting distance with variations in the load. However, the locus of the stop proved not always to be constant, and corrective downward movement was required on all loads lighter than the maximum load for which the system was adjusted.

The system of the present invention obtains the desired accuracy in stopping at the indicated landing, regardless of the prior speed of the elevator or of the load carried thereon. It has been found that it overcomes, in this regard, all of the objections which have been leveled against the systems of the prior art.

It is, therefore, a principal object of the present invention to provide a new and improved hydraulic elevator control system from which the objectionable aspects of the prior art systems have been eliminated.

Another object is to provide a new and improved hydraulic elevator control system incorporating a constant output pump, wherein an adjustable reduced speed is available for accurate landing.

Another object is to provide a new and improved hydraulic elevator control system wherein the above mentioned reduced speed is constant regardless of variations in the load being lifted.

Another object is to provide a new and improved hydraulic elevator control system wherein it is possible to make the reduced speed vary with the load in an arbitrary manner.

Another object is to provide a new and improvedl hydraulic elevator control system incorporating a flow regulating bypass valve which is capable of regulating pressure as well as flow.

Another object is to provide a new and improved hydraulic elevator control system wherein a predetermined portion of the output of a constant delivery pump is bypassed to obtain a reduced elevator ascending speed regardless of the load carried by the elevator.

Another object is to provide a new and improved hydraulic elevator control system wherein overload conditions are quickly'sensed and'relieved through a bypass valve.

' Still another object is to provide a new and improved hydraulic elevator system attaining the foregoing objects by equipment which is easily manufactured and not sensitive to friction of the working parts, or easily disarranged.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawings, wherein FIG. 1 is a diagrammatic View of a hydraulic elevator control system incorporating the novel features of the present invention, and showing the bypass valve and other valves in longitudinal section;

FIG. 2 is a diagram of the electric control circuit for the hydraulic system of FIG. 1;

FIG. 3 is a view similar to FIG. l, and showing a modified hydraulic control system and bypass valve; and

FIG. 4 is an expanded diagrammatic View of the flow regulating valve member of the bypass valves of this nvention.

In FIGS. 1 and 3, the bypass valve is shown approximately in the position it assumes when the elevator is ascending at slowed speed preparatory to making a stop at a landing.

Referring first to the form of the invention shown in FIG. 1, an elevator 10 is arranged to be raised and lowered by a hydraulic jack 12 having a cylinder 14 and piston 16 projecting from the upper end thereof and mounting the elevator 10 in the customary fashion. Fluid under pressure is supplied to the jack 12 by a constant volume output pump 18 which is driven by an electric motor 20, the inlet of the pump being connected by a conduit 22 to a hydraulic fluid supply tank or reservoir 24. The pump outlet is connected by a conduit or pipe 26, a check Valve 2S, and conduit 30 to the hydraulic jack 12. A lowering valve 32 has its inlet connected to the conduit 30 and its outlet connected to the reservoir 24 by a conduit 34. The lowering valve is of conventional construction known in this art and no detailed illustration or description is necessary.

A bypass valve 36 is interposed between the pump outlet and the tank 24 and has an inlet port 38 connected by a conduit 40 to the conduit 26 and an outlet port 42 connected by a conduit 44 to the conduit 34.

The ports 38 and 42 are formed in a valve body 220 and communicate respectively with an inlet chamber 222 and an outlet chamber 224, the chambers 2.22 and 224 communicating through a valve port or orice 226 with the fluid flow therebetween controlled by valve member 228 formed at one end of a valve spool 230.

The valve spool 230 has a piston 232 at the opposite end which is slidable in a cylinder 234 formed in the valve body 220. The open end of the cylinder is closed by a suitable cap 236 in which is fitted an adjustable stop screw 23S which limits the lift of the valve 22S and the travel of the valve spool to the right or in valve opening position. The valve is biased toward open position by a spring 240 which acts between the underside of the head of the piston 232 and a shoulder 242 formed at the junction between the cylinder 234 and the drain or outlet chamber 224.

The valve 228 is provided with valve wings 344 which center the valve 228 in the orifice 226 and with respect to the valve seat 246. Ports 248 (FIG. 4) between the wings 244 are specially shaped to provide the desired flow characteristic. The space between the wings is lled in adjacent the Vseat except for relatively narrow deep notches 250 which form the only passage for fluid between the chambers 222 and 224, when the valve 223 is in such position that the elevator is ascending at a slowed speed preparatory to stopping at a landing. When, however, the valve is in fully open position, the chambers 222 224 are communicated through the wider flow ports or passages 248.

The valve spool stem 249 is formed with an axial bore 252 in which is slidable a valve piston 254 between the lands of which is formed an annular groove 256. The

groove 256 communicates with an axial bore 258 in the piston 254 through radial ports 257, and therefore with the cylinder bore 252 in the valve spool 230. The cylinder bore 252 communicates with drain chamber 224 through a port 280. A passage 260 is drilled or cored in the valve spool piston 232 and stem 249 so that under certain circumstances the cylinder 234 will be drained through the passages 260 and 256, ports 257, passage 258, and port 280 to the chamber 224.

The valve piston 254 is connected by a stem 262 with a piston 264 slidable in a cylinder 266 formed in the valve body 220 and which is open to the inlet chamber 222. Movement of the piston 264 toward the inlet chamber is limited by a retaining ring 268 iitted into a suitable groove in the wall of the valve body 220.

The piston 264, stem 262, and valve piston 254 are biased to the right by a spring 270 bearing against a piston extension 272 integral with the piston 264 and of somewhat greater cross-sectional area than the stem 262, and projecting out of the valve body through bore 273. The force of the spring 270 is adjusted by a cap 274 which is threaded into a hollow boss forming chamber 276 in the projecting end of the valve body 220. An enclosing cap 278 prevents inadvertent adjustment of the adjustment member 274. The bore 258 extends through the piston 264 and extension 272 into the chamber 276 to relieve any pressure and drain any accumulated iluid through the port 280 to the reservoir 24.

The valve inlet 38 and inlet chamber 222 are connected to the closed end of the cylinder 266 behind the piston 264 through the port 288, conduit 284, adjustable needle valve 106, and conduits 118 and 115. A conduit 114 connects the conduit 118 and valve 106 to a normally closed valve 112, operable by solenoid 110. This valve controls ilow through conduit 116 which leads to the drain chamber 224 and the outlet 42.

The system also includes an adjustable needle valve 282 controlling flow through the conduit 284 and a conduit 286 connected directly to the cylinder 234. Thus, when the pump 18 is operating, uid will be transmitted to the cylinder 234, tending to close the valve 228 against the opposition of the spring 240.

With the pump 18 at rest the valve spool 230 is at the valve open position as dictated by the biasing spring 240 and the stop 238. When the motor 20 is started, the solenoid 110, controlling the valve 112, is energized to open communication between the conduits 115 and 114 and the conduit 116 to connect the cylinder 266 behind the piston 264 with the outlet chamber 224. The conduit 284 is also connected through the adjustable needle valve 106 and the branch conduit 118 to the conduit 114, and, when valve 112 is open, any flow through valve 106 is to drain. But when the valve 112 is closed, any iiuid ow from the inlet chamber 222 through the restricting needle valve 106 will be to the cylinder 266 to move the piston 264 to the right. However, when the elevator is ascending at maximum speed, the pressure in chamber 222 will move piston 264 to the left, emptying the cylinder 266 through the open valve 112.

This system is also provided with a high pressure relief pilot valve 290 including a valve body 292 and a piston type valve member 294 having a passage 296 therethrough. The valve piston 294 slides in a cylinder 298, and the head end of the piston 294 is exposed to the pressure at the inlet chamber 222 through the branch conduit 300 connected to conduit 284. The valve piston 294 is biased to the left by a spring 302, the tension on which may be adjusted by a bolt 304. When the pressure of the head end of the piston is great enough to overcome the force of the spring, the piston valve member 294 will move so that the passage 296 therein will connect conduit 306 leading to drain with conduit 308 connected to the conduit 286 and the head end of the valve piston cylinder 234 to allow the spring 240 rapidly to move the piston 232 and valve 228 to bypass open position, thus relieving the overload at the valve inlet 38.

When it is desired to raise the elevator, the circuit to the motor 20 is closed, energizing the motor to operate pump 18, and the solenoid controlled valve 112 is also opened. Since, at the start of a raising sequence, the valve 228 is in its fully open position, uid will ow from the pump 18 through the inlet chamber 222, the valve orifice 226, and the drain chamber 224 to the tank 24. However, the lift of the valve 228 is such that full flow therethrough produces a pressure rise at the inlet 38 and in the chamber 222, so that some uid will ilow through the conduit 284, needle valve 282, and conduit 286, to the cylinder 234, thus producing a pressure rise therein to urge the piston 232 to the left and against the force of the biasing spring 240. This also moves the valve 228 toward closed position and as it approaches closed position the pressure in the chamber 222 will rise until it is high enough to open the check valve 28, thereby delivering iluid to the jack 12 to raise the elevator 10. The pressure rise in the cylinder 234 continues until the valve 228 closes. Since the valve 112 has been opened by solenoid 110, the pressure rise in chamber 222 moves piston 264 to the left to drain the cylinder 266. This movement, of course, prevents communication between the passage 260 in the piston 232 and the passages 256 and 258 in the valve piston 254 and stem 262, thereby preventing the drainage of iluid coming into the cylinder 234. Because the Valve 112 is open when the elevator is ascending at full speed, the relatively small inilow through the conduit 284 and the needle valve 106 is drained away Without appreciable pressure rise in the cylinder 266.

When the elevator reaches a predetermined point in the travel where slowdown is desired in preparation for a stop, the circuit to the solenoid 110 is opened, thereby closing the valve 112 and terminating the escape of iiuid from the cylinder 266 and the pressure tends to equalize in the cylinder 266 and in the inlet port 38 and chamber 222. Whereas the pressure in inlet chamber 222 acting on the right face of the piston 264 during full speed ascent was resisted only by the spring 270, thus maintaining the piston 264 at its left-most position, the pressure buildup in the cylinder 266 augments the force of spring 274 moving the piston 264 to the right. Nevertheless, inasmuch as the area of the left face of piston 264 adjacent extension 272 is less than the area of the right face, the net force due to pressure acting on the piston 264 is t0 the left. Accordingly, the piston 264 moves to the right until the force of spring 270 balances the net force on piston 264 due to pressure, which is proportional to the working load on the elevator car. The force of spring 270 is so proportioned that the pressure in chamber 222 and cylinder 266 corresponding to minimum working load moves the piston 264 to the right until it stops only a little way from its stop ring 268 while the pressure corresponding to the heaviest load will move the piston 264 only a small distance from its left-hand stop. The rate at which the piston 264 moves to the right is adjustable by the setting of the needle Valve 106 which regulates the rate at which iluid is admitted to the cylinder 266 behind the piston 264. When the piston 264 comes to rest, pressure in the cylinder 266 will be equal to that in the inlet chamber 222.

Movement to the right of the piston 264 when the valve 228 is fully closed uncovers the passage 260 to communicate it with the passage 256, 258, allowing fluid t0 escape from the cylinder 234 to the outlet chamber 224, and allowing the valve to open under the pressure of the spring 240. Opening of the valve 228 will continue until the passage 260 has only restricted communication with the passage 256, and the ow of iluid passing the valve 282 will exactly equal that escaping through the port 260, passage 256, to drain.

When the elevator cage is substantially at the landing, a limit switch is operated to break the circuit to the motor 20, thereby stopping the pumping operation and permitting the spring 240 to move the piston 232 to the right and the spring 270 will move the piston 264 tov f the right. Since the latter will move more rapidly than the former, the port 260 will communicate with the passage 256 to drain the cylinder 234. The system therefore would be conditioned for a starting operation at maximum pump output.

The control system also includes certain electric controls for the solenoid 110 and the motor 20, which are shown in the wiring diagram of FIG. 2. Power is supplied to the motor 20 and to the control circuit from a three-phase 220 volt source 178. The control circuit receives its power through the lines 180 and 182 connected to one side of the power source 178, as is conventional. A master gang switch 184 controls the motor and the entire system, and the motor is controlled directly by a series of motor switches 186.

Up circuit 192 includes a manually operated switch 188 located in the elevator cage or at the operators stand, a normally closed top position limit switch 190, a normally closed down switch DR2, an up operating relay UR, and the solenoid 110 in parallel with the relay, all of which are in the line connecting the lines 130 and 182. When the switch 188 is closed, current flows from the line 180 to the line 182 through the closed switch 188, upper limit switch 190, switch DR2, and relay UR and also the solenoid 110, thus opening the solenoid operated valve 112. Energization of the relay UR closes the switch URI in circuit 191 to the motor starter relay R20. The relay R20 closes the switches 186, thereby energizing the circuit to the motor 20, causing it to drive the pump 18 to displace iluid therefrom. Closing of the relay UR also opens a switch UR2 in the circuit 194 to the down switch 196 and down relay DR, the latter relay controlling the operation of the lowering valve 32, thereby rendering this latter circuit inoperative to operate the valve 32 at any time while the elevator is ascending. When the elevator nears the landing, a cam 198 on the elevator cage will open the switch 190, thereby deenergizing the relay UR to allow the switch UR1 to open and deenergize the solenoid 110 to permit the valve 112 to close.

When the relay UR was energized, it opened a switch UR3 in circuit 200 which includes a solenoid 201 operating the lowering valve 32.

As the elevator approaches the landing, the limit switch 190 is opened or the manual switch 188 is released, but up leveling switch 202 on the cage engages a stationary cam 204 in the hoisting structure. The switch 202 is normally open, but as it engages the cam 204 it closes to energize the relay ULR in the circuit 206 between the lines 180 and 182. Energization of the relay ULR closes the switch ULR1 which is in the circuit 208 in parallel with the normally and now open switch URI, and in series with normally closed switch DRI (opened by relay DR), and thereby maintaining the motor relay R energized. When the elevator reaches the landing, the up leveling switch 202 opens, breaking the circuit to the relay ULR which in turn allows switch ULR1 to open and deenergize the motor relay R20 to stop the motor and elevator.

When the elevator is rising under full speed conditions, it closes down leveling switch 214 in circuit 215 to down leveling relay DLR to engage the circuit and relay, thereby to close the switch DLR1 in circuit to solenoid 201. Since switch URS is held open by energized relay UR, the solenoid is not energized and this action has no effect on the system. Should the elevator close an up leveling switch 202 at a landing where no stop is contemplated, the relay ULR will be energized but will have no effect on the operation of the elevator because the bypass valve is closed. Should the elevator settle at the landing, the switch 202 will close, thereby energizing the relay ULR to close the switch ULR1 and to energize the motor starting relay to restore the elevator to the landing.

When it is desired to lower the elevator, the manually operated down switch 196 in the circuit 194 is closed. This circuit includes the normally closed switch UR2 which has been closed, upon deenergization of the relay UR and the down operated relay DR, which opens the switch DRI in series with the up leveling switch ULR1 and the switch DR2 in series with the relay UR. The circuit to the motor operating relay R20 is deenergized so that the motor 20 will not be energized when the elevator closes an up leveling switch 202 during descent, and the up circuit 192 is also deenergized. The circuit 194 also includes the bottom limit switch 210 which is opened by the cam 198 as the elevator approaches the bottom of its travel and is closed as the elevator leaves the bottom position. Closing of the switch 196 energizes the circuit to the relay DR and a solenoid 212 to open the lowering valve 32, which is of conventional design so that the lowering speed of the elevator is automatically regulated.

If the elevator should rise above a landing at which it has stopped, a down leveling switch 214 will be closed by the shaft cam 204. The switch 214 energizes relay DLR in the circuit 215 to close a switch DLRl through the normally closed switches DRI and UR3 in the circuit 200 to the solenoid 201 to operate the lowering valve 32 to meter a small amount of fluid from the jack to the reservoir 24, thereby lowering the elevator to the desired level.

The circuit is under the control of a master manual switch 216 and a pair of maximum top and bottom limit switches 218 which are in the lines and 182, and the functions of which are obvious.

The form of the system shown in FIG. 3, where the same characters of reference are used to indicate similar parts, the head end of the cylinder 234 is closed by a partition 310, and the partition and a cap 312 are secured to the valve body 220 by conventional bolts 314 and sealed against leakage by gaskets 316. The movement limiting stop 238 is carried by the piston 232 and is arranged to abut the partition 310. The piston 232 is formed with an axial bore 318 extending into the stem 262 and is counterbored at 320 to provide a valve cylinder.

A valve rod 322 having a stem 324 of smaller diameter slides, respectively, in the bores 320 and 318. The valve rod 322 is provided with a plurality of longitudinally extending circumferentially arranged ports 326 in communication with a port 328 in the piston stem 262, so that under certain circumstances the drain chamber 224 may be communicated with the head end of the cylinder 234 via the communicating ports 326 and 328.

The valve stem 322 projects through a closely fitting hole 330 in the partition 310 and is integral with a piston 332 slidable in a cylinder 334 formed in the cap 312. The rightward movement of the cylinder is arrested by a stop 336 formed in the cap and a biasing spring 338 tends to move the piston 332 toward the stop 336. Maximum leftward movement of the piston is determined by an enlarged portion or stop 340 formed as an enlargement of the valve stem 322, and it is arranged to abut the partition 310. An axial bore 342 extends through the piston 332 and valve stem 322 to communicate the passage 318 with chamber 344 at the head end of the cylinder 334. This passage extends through and communicates with the inlet side of the valve 228 so that the pressure in the chamber 344 will be at inlet chamber pressure.

The cylinder 334 and the rod of the piston 332 form a chamber 346.

The inlet chamber 222 is connected to the needle valve 106 by a conduit 348, and the valve 106 is connected to the solenoid operated valve 112 by a conduit 358. The outlet of the valve 112 is connected to the drain port or chamber 224 by conduit 360. The chamber 346 is connected to the conduit 358 between the valves 106 and 112 by a conduit 362.

Needle valve 282 is connected to the conduit 348 by branch conduit 364 and to the head end of the cylinder 234 by conduit 366. The conduit 366 is connected to the pressure relief valve 290 by a conduit 368 so that at certain times it may be in communication with the passage 296 in the valve spool 294, to drain the cylinder 234 and relieve excess pressure at the inlet port 40. Conduit 360 is also connected to the valve 290 by a conduit 370 to effect the desired drainage through the pressure relief valve.

In this system the operation is essentially the same as that shown in FIG. 1, with the piston 332 corresponding in its function to the piston 264 in communicating the head end of the cylinder 234 with the drain through the ports 326 and 328.

It will be seen from the foregoing description that the objectives which were claimed for this invention have been attained by the various structures shown and described.

While two embodiments of the hydraulic elevator control system constituting this invention have been shown and described, it will be apparent that other modifications thereof may be made without departing from the underlying principles of the invention. It is therefore desired, by the following claims, to include within the scope of the invention all such variations and modifications by which substantially the results of the invention may be obtained through the use of substantially the same or equivalent means.

What is claimed as new and desired to be secured by United States Letters Patent is:

1. A bypass valve for a hydraulic control system for hydraulic lifting devices having variable load conditions, having a source of fiuid, and a constant output pump with its inlet connected to the fluid source and its outlet connected to the lifting device, said bypass valve comprising in combination an inlet port and an outlet port, said inlet port being adapted to be connected to the outlet of the pump and said outlet port being adapted to be connected to the fluid source, an orifice in said valve connecting said inlet port with said outlet port, a valve member controlling the flow of fluid between said ports and the amount of fluid bypassed, means biasing said valve member to open position, piston means connected to said valve member and arranged to move said valve member toward closed position, means including a restriction connecting said valve inlet to said piston means to apply high pressure fluid to said piston means to move the latter in valve member closing direction, second piston means arranged for movement in one direction by pressure of fluid at said inlet port, means biasing said second piston means in a second direction, means including a restriction connecting said valve inlet to said second piston means to apply a pressure tQ the latter to augment said second piston biasing means, controllable means bypassing said last named means, and valve means controlled by relative movement of both said piston means for relieving the pressure applied to said first piston means.

2. A bypass valve for a hydraulic control system for lifting devices having variable load conditions, having a source of uid, and a constant output pump with its inlet connected to the fluid source and its outlet connected to the lifting device, comprising an inlet adapted to be connected to the output of the pump and an outlet adapted to be connected to the fluid source, a valve member controlling uid flow between said inlet and said outlet, means for continuously biasing said valve member toward an open position, pressure operated means connected to said valve member for moving said valve member toward closed position, means including a restriction connecting said valve inlet to said pressure operated means for applying fluid under pressure thereto to move said valve member to closed position, second pressure operated means selectively responsive to the fluid pressure at said valve inlet, and means operated by both said pressure operated means for partially relieving the pressure applied to said first pressure operated means, thereby to hold said valve member in a partially open position to bypass a predetermined quantity of fluid from the said valve inlet to said valve outlet.

3. A bypass valve for a hydraulic control system for lifting devices having variable load conditions, having a source of uid, and a constant output pump with its inlet connected to the fluid sourcce and its outlet connected to the lifting device, comprising an inlet adapted to be connected to the output of the pump and an outlet adapted to be connected to the Huid source, a valve member controlling fluid flow between said inlet and said outlet, means for continuously biasing said valve member toward an open position, first pressure operated means connected to said valve member for moving said valve member toward closed position, means including a restriction connecting said valve inlet to said pressure operated means for applying huid under pressure thereto to move said valve member to closed position, valve means for bleeding a portion of the fluid applied to said pressure operated means to said outlet, ysecond pressure operated means, said valve means being connected to and operated by both said pressure operated means, means adapted to be controlled by the position of the lifting device during ascent for selectively applying pressure to said second pressure operated means to cause said valve means to bleed a predetermined amount of fluid from said first pres- Sure operated means, whereby said biasing means will move said valve member to partially open position to bypass a predetermined portion of the uid to said valve outlet.

4. A bypass valve for a hydraulic lifting device control comprising a valve body having an inlet adapted to be connected to the output of a hydraulic pump and an outlet adapted to be connected to drain, a bypass valve orifice communicating said inlet with said outlet, a variable flow valve member movable in said orifice between opcn and closed positions and controlling the ow of fluid therethrough, means for continuously biasing said valve member toward an open position, means forming a first cylinder and piston combination connected to said Valve member and provided with means including a restriction connecting said cylinder to said inlet to overcome the force of said biasing means to move said valve member toward closed position, means forming a second cylinder and piston combination, said second piston being movable in a first direction by the pressure of fluid at said inlet, means biasing said second piston in a second direction, and valve means connected to and operable by said pistons connecting said rst cylinder to drain so that said first biasing means may move said valve member to partially open position.

References Cited in the file of this patent UNITED STATES PATENTS 1,071,777 Murphy Sept. 2, 1913 2,553,045 Jaseph May 15, 1951 2,737,197 Jaseph Mar. 6, 1956 2,835,266 Morte May 20, 1958 2,944,401 Beck July 12, 1960 

2. A BYPASS VALVE FOR A HYDRAULIC CONTROL SYSTEM FOR LIFTING DEVICES HAVING VARIABLE LOAD CONDITIONS, HAVING A SOURCE OF FLUID, AND A CONSTANT OUTPUT PUMP WITH ITS INLET CONNECTED TO THE FLUID SOURCE AND ITS OUTLET CONNECTED TO THE LIFTING DEVICE, COMPRISING AN INLET ADAPTED TO BE CONNECTED TO THE OUTPUT OF THE PUMP AND AN OUTLET ADAPTED TO BE CONNECTED TO THE FLUID SOURCE, A VALVE MEMBER CONTROLLING FLUID FLOW BETWEEN SAID INLET AND SAID OUTLET, MEANS FOR CONTINUOUSLY BIASING SAID VALVE MEMBER TOWARD AN OPEN POSITION, PRESSURE OPERATED MEANS CONNECTED TO SAID VALVE MEMBER FOR MOVING SAID VALVE MEMBER TOWARD AN OPEN POSITION, PRESSURE OPERATED MEANS CONNECTED TO SAID VALVE MEMBER FOR MOVING SAID VALVE MEMBER TOWARD CLOSED POSITION, MEANS INCLUDING A RESTRICTION CONNECTING SAID VALVE INLET TO SAID PRESSURE OPERATED MEANS FOR APPLYING FLUID UNDER PRESSURE THERETO TO MOVE SAID VALVE MEMBER TO CLOSED POSITION, SECOND PRESSURE OPERATED MEANS SELECTIVELY RESPONSIVE TO THE FLUID PRESSURE AT SAID VALVE INLET, AND MEANS OPERATED BY BOTH SAID PRESSURE OPERATED MEANS FOR PARTIALLY RELIEVING THE PRESSURE APPLIED TO SAID FIRST PRESSURE OPERATED MEANS, THEREBY TO HOLD SAID VALVE MEMBER IN A PARTIALLY OPEN POSITION TO BYPASS A PREDETERMINED QUANTITY OF FLUID FROM THE SAID VALVE INLET TO SAID VALVE OUTLET. 